Method of displacement chromatography

ABSTRACT

A plurality of chromatographic separation operations, including a first simulated moving bed operation, are coupled into a process which functions, preferably through the application of continuous displacement chromatography, to recover a fraction rich in small organic molecules, notably betaine and/or invert from sucrose solutions, enabling the subsequent production of a high purity sucrose product.

REFERENCE TO RELATED APPLICATIONS

This application is a 371 of PCT/US98/01512 filed Jan. 28, 1998 andclaims the benefit of U.S. Provisional Application No. 60/036,603 Jan.29, 1997.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention pertains to a process of simulated moving bedchromatography. It is particularly directed to operation of a simulatedmoving bed in coupled relation to a second chromatographic separationprocess. It provides for the recovery of a betaine and/or invertfraction from sugar solutions and the coupled production of a highpurity sucrose product.

2. Background Art

U.S. Pat. No. 4,412,866 describes the operation of an SMB to separatethe components of a feed stock. A resin bed is divided into a series ofdiscrete vessels, each of which functions as a zone within a circulationloop. A manifold system connects the vessels and directs in appropriatesequence to (or from) each vessel each of the four media accommodated bythe process. Those media are generally referred to as feed stock,eluent, extract and raffinate, respectively. As applied to a sugarfactory, a typical feed stock is sucrose solution, the eluent is water,the extract is an aqueous solution of sucrose and the raffinate is anaqueous solution containing nonsucrose, such as salts and high molecularweight compounds. The SMB disclosed by the '866 patent is of the typesometimes referred to as a “continuous SMB,” to distinguish it fromanother type, sometime referred to as a “sequential SMB.” Unlessotherwise indicated, the term “SMB” is used in this disclosure to denotea continuous SMB.

The largest single loss of sugar values from a typical sugar factory isattributable to molasses formation. Molasses comprises the byproduct (orwaste) stream remaining after repeated crystallization procedures areapplied to recover purified sugar. This molasses is typically of suchlow purity that further crystallization procedures for the recovery ofadditional sugar are economically impractical. SMB arrangements similarto those disclosed by the '866 patent are used in sugar factories toprocess molasses; typically producing a product fraction of relativelyhigh (e.g., 90%) purity and low ash content and a byproduct fraction,comprising, typically, 40-50% of the feed, of relatively low purity andlow ash content. (As used in the sugar industry, “purity” specifiespercent by weight sucrose of the solids contained in a sample, on a dryweight basis.)

In the sugar beet industry, the byproduct fraction contains most of thebetaine values of the molasses feed. Betaine, being the most abundantnitrogenous compound found in molasses, has been recognized as acommercially useful byproduct; notably for use in animal feeds. In thesugar cane industry, the byproduct fraction contains most of the invertsugar (i.e., glucose and fructose) values of the molasses feed. Theinvert is a valuable digestible carbohydrate. As used herein, the term“invert” refers to “invert sugar” (a mixture of glucose and fructoseformed in equal quantities by the hydrolysis of sucrose).

In the typical operation of SMB chromatography, the product sucrosefraction (extract) is contaminated to some extent by betaine and/orinvert. Such contamination reduces the recovery of these valuablebyproducts and reduces the purity of the sucrose product. Thisdisadvantage is attributable to the steps inherent in typical SMBoperation. The SMB is initially inventoried with solids to anequilibrium state, and thereafter, feed and eluent are fed into thecontinuously recycling inventory while extract and raffinate arewithdrawn from the recycling inventory. In the context of thisdisclosure, the term “inventory” refers to the distribution and identityof chemical species constituting the recycle stream. This recycling ofinventory is generally a very favorable aspect of the SMB, becausematerial is subjected to a very long chromatographic path forseparation; dependent upon chosen recycle rates and inventory levels. Asa result, difficult-to-remove materials, such as certain colorcompounds, are efficiently removed. In comparison, inventory build-upand long separation path length (due to circulation) are inconsistentwith batch chromatography. Eluent use is also much lower in SMBchromatography because the eluent is continuously recycled (eluentphase) as part of the internal inventory.

Unfortunately, providing a continuous, quickly recycling, internalconfiguration results in only two well separated components, one on eachend of the separation profile. Some nonsucroses, such as ash and highmolecular weight compounds, move much more quickly than sucrose throughthe monovalent form ion exchange separation resins typically employed,and therefore move to the front of the recirculating separation profile.Some other nonsucroses, such as betaine, invert and certain amino acids,move much more slowly than sucrose through these resins. As aconsequence, either the faster moving nonsucroses or the slower movingnonsucroses must always be crossing through the separated sucrose,thereby contaminating the sucrose.

The extract obtained from the chromatography of sucrose solutions isconventionally subjected to crystallization procedures, resulting in anacceptably pure saleable sugar product. The highest present daycommercial standards for overall recovery of crystallized sucrose frombeet molasses suggest a chromatography efficiency of 92 purity sucroseat 90% recovery. At this purity level, subsequent recovery bycrystallization procedures is typically about 87%, with a loss of somesucrose to a second molasses (60 purity). Overall, a “superior” combinedresult of conventional chromatography and crystallization procedures hasbeen the recovery of up to about 78% of the sucrose content of theinitial molasses as crystalized sucrose.

U.S. Pat. Nos. 4,359,430 and 5,127,957 describe methods for the recoveryof a betaine fraction from various sources, such as the molassesproduced by a sugar factory. The '957 patent discloses a discontinuouscirculation, batch-wise operation. The method involves shutting off allinput and output streams to the SMB while maintaining circulationthrough the resin bed. Thereafter, circulation is halted, and waterand/or molasses feed are introduced at specified locations to displacebetaine, sucrose and rest molasses from separate columns in the loop.The “circulation” taught by the '957 patent is not a true recycleconventional to continuous SMB systems; it merely functions to displacethe separation profile to an assigned location in the resin bed. Bycontrast, the continuous recycle stream, which is essential to acontinuous SMB operation, circulates the separation profile continuouslythrough the resin bed.

International Application WO 96/10650 describes a proposedbetaine-recovery process which accepts the conventional 92 puritystandard as the applicable goal for the sucrose fraction of the process.The WO 96/10650 process contains no suggestion that a sucrose fractionfree from significant quantities of betaine or other small organicmolecules be collected.

Ordinarily, the chromatography of sucrose containing mixtures, such assugar beet molasses, involves variations of elution chromatography. Thefeed mixture enters a chromatographic configuration of some type, anddue to a preferential adsorption, the sucrose product is collectedsomewhere roughly from the middle to the trailing edge of the developedelution profile. As an elution system is loaded higher and higher withfeed mixture, or as eluent is progressively reduced, the bands ofseparated material broaden and increasingly overlap. As a consequence,separation deteriorates. Inevitably, the efficiency of elutionchromatography is limited by mixture loading and elution volume.

It is understood by those skilled in batch chromatography (as opposed tosimulated moving bed chromatography) that very high column loading underappropriate conditions provides potentially advantageous specificeffects. At large sample load, the components of the mixture to beseparated can interfere, and the elution peaks can be modified. Afavorable case occurs when the most retained component has the highestsaturation capacity. In that case, the most retained component pushesthe least retained component ahead of it, and the separation canactually be much better than could be expected from elution mechanisms.Because of this “displacement effect,” high load separation can actuallybe superior to separation under elution conditions. When gooddisplacement conditions are obtained, a batch column can be overloadedbeyond the level expected from evaluation of data determined withanalytical chromatograms of individual components. In batch displacementchromatography, the retained component with high saturation capacity,referred to as the “displacer,” is most often a molecule chosen for itsuseful displacement characteristics, and is not necessarily a componentof the mixture to be separated.

There remains a need for an improved SMB method for purifying sucrosesolutions wherein the sucrose containing product is not contaminated bycross-over nonsucroses, the betaine and/or invert are recovered almostcompletely, and the advantages of SMB chromatography are notcompromised.

DISCLOSURE OF INVENTION

In general terms, this invention constitutes an improvement to asimulated moving bed process for the recovery of a first productfraction predominating in a first product species from a liquid mixturecontaining that first product species in admixture with a secondbyproduct species. A feed stock is introduced to a recycle streamcirculating through a partitioned bed of resin, raffinate is withdrawnfrom the recycle stream downstream from the introduction of the feedstock, eluent is introduced to the recycle stream downstream from thewithdrawal of raffinate, and extract is withdrawn from the recyclestream downstream from the introduction of eluent, all in usual SMBfashion. The feed stock usually comprises a relatively large amount ofthe product species and a relatively small amount of the byproductspecies.

The improvement of this invention generally comprises establishing acontinuous simulated moving bed system in which a feed stock, comprisinga relatively large amount of a product species and a relatively smallamount of a byproduct species, is fed into a recycle stream circulatingthrough a partitioned bed of resin, the recycle stream beingcharacterized by a separation profile. A raffinate, comprising separatedproduct species, and containing other contaminant species, is removedfrom the recycle stream in the vicinity of the leading edge of theseparation profile, downstream from the introduction of feed stock.Eluent is introduced to the recycle stream downstream from thewithdrawal of raffinate, and an extract, comprising separated byproductspecies, is withdrawn from the recycle stream downstream from theintroduction of eluent, in the vicinity of the trailing edge of theseparation profile. A significant improvement constitutes accumulatinginto the recycle stream sufficient displacer species to displaceportions of the product species toward the leading edge of theseparation profile of the recycle stream. Ideally, the displacer speciesis the byproduct species.

From another view, in the operation of a continuous simulated moving bedsystem to separate the components of a feed stock, wherein a resin bedis divided into a series of discrete vessels, each of which functions asa zone within a circulation loop through which is maintained arelatively large-volume continuous recycle stream, and a manifold systemconnects the vessels and directs in appropriate sequence to each suchvessel relatively small-volume streams of feed stock and eluent,respectively, and from each vessel relatively small-volume streams ofextract and raffinate, respectively, whereby to develop a circulatinginventory of chromatographically separated chemical speciescharacterized by a separation profile including an intermediate regionoccupied by a commercially valuable phase bounded by a trailing regionand a leading region, the invention offers an improvement whichcomprises operating the system to establish a high inventory of atrailing separated species, whereby to displace a commercially valuableseparated species into that leading region, and collecting a raffinatestream from that leading region of the separation profile of the recyclestream.

According to certain practical embodiments of this invention, betaineand/or invert is removed from sucrose solutions via SMB chromatographyin near totality prior to purification of the sucrose. These twoseparations are ideally conducted in coupled SMB systems, with theraffinate produced by the first SMB (SMB A) being processed by a secondSMB (SMB B). Independent and different operating parameters (appropriatefor each SMB), independent inventory profiles, and independentcontinuous internal recycle loops are employed for these two respectiveoperations.

The advantageous functions of SMB chromatography are insured bymaintaining inventory build-up and continuous internal circulation oneach SMB. The betaine and/or invert are recovered in near totality froma first extract fraction. The sucrose product, recovered as an extractfrom the raffinate fraction resulting from recovery of the first extractfraction, is of substantially higher purity than that produced byconventional operation due to the near total elimination of thecrossover nonsucroses (betaine/invert).

In general, this invention may be viewed as an improvement over asimulated moving bed process for the recovery of sucrose from a rawsugar solution in which a feed stock is introduced to a recycle streamcirculating through a partitioned bed of resin, raffinate is withdrawn(as a small-volume “sample”) from that recycle stream downstream fromthe introduction of the feed stock, a compensating volume of water isintroduced to that recycle stream downstream from the withdrawal of theraffinate, and a balancing “sample” amount of sucrose is withdrawn fromthat recycle stream downstream from the introduction of water. Thepurity of the sucrose recovered from SMB processing is enhanced inaccordance with this invention by establishing first and secondchromatographic procedures, the first of which is a continuous SMB, andthe second of which can be of any convenient chromatographicconfiguration, including continuous SMB, sequential SMB or batchconfigurations. Continuous simulated moving bed configurations arecurrently preferred for both the first and second chromatographicprocedures, and this disclosure makes primary reference toconfigurations which couple two continuous simulated moving beds,designated for convenience, SMB A and SMB B.

SMB A is operated such that a first feed stock, comprising sucrose andnonsucroses, is fed into a first recycle stream circulating through afirst partitioned bed of resin; a first raffinate, comprising sucrose,ash and high molecular weight compounds separated from the feed stock,is removed from the first recycle stream downstream from theintroduction of the first feed stock; water is introduced to the firstrecycle stream downstream from the withdrawal of the first raffinate;and a first extract, comprising nonsucroses separated from the firstfeed stock, is withdrawn from the first recycle stream downstream fromthe introduction of water. A second simulated moving bed, SMB B, isoperated such that a second feed stock, comprising the first raffinate(from SMB A), is fed into a second recycle stream circulating through asecond partitioned bed of resin; a second raffinate, comprising ash andhigh molecular weight compounds separated from the first raffinate, isremoved from the second recycle stream downstream from the introductionof the second feed stock; water is introduced to the second recyclestream downstream from the withdrawal of the second raffinate; and asecond extract, comprising sucrose, is withdrawn from the second recyclestream downstream from the introduction of water.

Broadly, the invention can be viewed as a type of “displacementchromatography” applied to a simulated moving bed operation. Thismechanism results in remarkable and unexpected improved efficiency ofseparation. According to this invention, forcing a very high steadystate inventory of small trailing organic molecules in a simulatedmoving bed causes a useful “displacement effect.” Unlike batchdisplacement chromatography, the displacer is maintained in anequilibrated state, thereby providing a type of “continuous displacementchromatography.” With sucrose mixtures, such as sugar beet or sugar canesolutions, the displacer is ideally part of the feed mixture, ratherthan an added component. The practice of the novel continuousdisplacement chromatography of this invention offers several unique andadvantageous characteristics when applied to sucrose solutions. Forexample:

1. Because displacement rather than elution is key to the separationmechanism, the quantity of eluent used to separate the small organicmolecules from the sucrose is very low. Water added to a chromatographicseparation process must generally be recovered. Thus, a low requirementfor water addition is economically advantageous. Typical molasseschromatographic separation systems use ratios of about 6.0-8.0 watervolume for each volume of feed molasses (assuming the feed is 60%dissolved solids). By contrast, the displacement procedure of thisinvention permits the small trailing organic compounds to be separatedalmost entirely from the sucrose with water-to-feed ratios of 2.0 orless.

2. The displacement effect results in greater than 90% separation of thetrailing small organic molecules, such as betaine, from the sucrose.These compounds are usually difficult to separate from the sucrose inindustrial scale operations.

3. The concentration of the small organic molecule fraction is very highcompared with the corresponding fraction recovered through elutiontechniques. Assuming an initial molasses of 60 purity and 60% dissolvedsolids, elution techniques typically yield betaine containing fractionsof 1% to 5% dissolved solids. With the disclosed displacement method,typical betaine-containing fractions contain about 8% to 15% dissolvedsolids.

4. The displacement effect forces a very high recovery of the sucrose,typically greater than 99%. It has not previously been recognized thatbuilding a large inventory of nonsucrose in a chromatographic separatorwill improve the sucrose recovery.

5. In conventional industrial sucrose elution chromatography, withstrong cation ion exchange resins as the stationary phase, sucrose is apreferentially adsorbed component. As a result, the sucrose is collectedsomewhere roughly from the middle to the trailing edge of the developedelution profile. In contrast, the displacement procedure of thisinvention results in sucrose being pushed in exactly the oppositedirection—into the area of the leading edge. The key advantage of thiscontradictory effect is that the sucrose is collected withconventionally easily separated leading edge components such as ash andvery high molecular weight compounds. The coupled simulated moving bedaspect of this invention is therefore greatly enhanced because thedifficult-to-remove small organic nonsucroses are absent following afirst SMB so that a subsequent second SMB operating in a conventionalmanner can recover the sucrose at extremely high purity. Purities offinal sucrose fractions from sugar beet molasses have been observed toexceed 97% on dissolved solids. High purity products are generallyadvantageous, and in this case, the benefits include improved processingcharacteristics, such as faster crystallization kinetics and lowerproduct odor.

6. The final sucrose fraction is typically subjected to crystallizationto obtain the sucrose as a pure product along with a second molasses.The new process of this invention is capable of producing a 96% puritysucrose fraction at 96% recovery so that the overall recovery of productsucrose after the combined operations of chromatographic separation andcrystallization increases from the present practical standard of 78% toa new practical standard of about 90%.

7. Simulated moving bed operations require a certain length of time(number of cycles) to equilibrate. With the procedure of this invention,equilibration time can be reduced by adding previously stored orprepared small organic molecule fraction to the feed material or to anappropriate location in the chromatographic separator.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which illustrate what is currently regarded as the bestmode for carrying out the invention:

FIG. 1 is a simplified flow diagram of an SMB system typical of aconventional ion exclusion process;

FIG. 2 is a simplified flow diagram of a coupled SMB system which allowsfor realization of this invention; and

FIGS. 3 and 4 are drawings of chromatographic solids profiles pertinentto an embodiment of this invention.

BEST MODES FOR CARRYING OUT THE INVENTION

As best illustrated by FIG. 1, eight cells (each comprising one or morevessels) are arranged to receive flow in series, all as explained indetail in U.S. Pat. No. 4,412,866. The cells are designated 1 through 8respectively. A recycle, or circulation, stream flows continuously at a“basic” flow rate through the loop including the cells, 1-8, and aconnecting manifold system, designated generally 11. The manifold 11 isconfigured to introduce feed stock F to the circulation stream exitingcell 8 as it flows to the top of cell 1, to withdraw raffinate R fromthe circulation stream exiting the bottom of cell 2, to introduce waterW to the circulation stream entering the top of cell 5 and to withdrawextract E from the circulation stream exiting cell 6. These streamsdivide the total resin bed in the system into four zones, each of whichcomprises at least one cell. The extract stream contains the purifiedsucrose with typical betaine and/or invert contamination. The raffinatestream contains the remaining betaine and invert, ash and high molecularweight compounds. Typically, the resin used for the chromatographicseparation is a monovalent form strong cation ion exchange resin.

In a typical SMB system, the manifold system 11 operates to shift eachinput, F and W, and each of the outputs, R and E, as a group to otherlocations around the loop. The specific SMB system illustrated iscontrolled through 8 steps. Accordingly, feed stock F will, in turn, beintroduced in sequence to the tops of each of columns 1 through 8, whileeach of the other input and output streams will be similarly advanced tomaintain their respective positions in the loop.

The arrangement illustrated by FIG. 2 allows for the recovery of betaineand/or invert prior to sucrose purification. In SMB A, designatedgenerally 12, feed stock F1 is introduced to the circulation loopexiting cell 4 while simultaneously water W1 is introduced to thecirculation loop exiting cell 2, extract E1 is removed from thecirculation loop exiting cell 3 and raffinate R1 is removed from thecirculation loop exiting cell 1. F1 is a sucrose solution containingbetaine and/or invert components. E1 is an extract stream, and containsthe separated betaine and/or invert components. R1 is a raffinatestream, and contains the separated sucrose, ash and high molecularweight compounds. It is noted that in the usual operation of a simulatedmoving bed, e.g., as described with reference to FIG. 1, the sucroseexits with the extract. In this respect, SMB A of this inventionoperates in a reverse manner as compared to conventional methods.

In SMB B, designated generally 13, operation is generally as describedwith reference to FIG. 1 except that feed stock F2 consists of raffinatefrom SMB A, extract E2 is sucrose product separated from ash and highmolecular weight compounds, and raffinate R2 is the separated ash andhigh molecular weight compounds.

Although any number of separator cells appropriate for simulated movingbed operation may be used for either SMB A or SMB B (generally morecells yield improved separation), the betaine/invert separation providedby SMB A is sufficiently efficient that the four cells illustrated aregenerally adequate for this operation. Additional cells would ordinarilyadd to the cost of the process, without a corresponding benefit.

Concentrator 14, while not required is a preferred component of thesystem. A high percent dry substance (% DS) feed is known to result inimproved sucrose recovery. The concentrator 14 is thus desirablyemployed to raise the dry substance content of the raffinate from SMB Afrom a starting value of typically 25%-35% DS to a final value withinthe range of about 40% to about 70% DS before entering SMB B. Theconcentrator 14 can be of any conventional type, typically an evaporatorsystem.

EXAMPLE 1

An equipment configuration as illustrated in FIG. 2 was evaluated. SMB Awas operated using the disclosed displacement chromatography method. The30 minute step time on SMB A was split into two periods of 15 minuteseach. Recirculation was continuous during both periods, while feed,water, extract and raffinate were continuous during period 1. Such aconfiguration is useful to improve the efficiency of a four cell SMBoperation and is described in U.S. Pat. No. 5,102,553. Such a method isnot generally beneficial when more cells are used for SMB A. SMB B wasoperated with all flows continuous (recirculation, feed, water, extractand raffinate). The following operating parameters were used:

SMB A SMB B Feed material beet molasses raffinate from SMB A Feed (% DS)60 60* Number of cells  4  8 Separation resin Dow 350 micron Dow 350micron monovalent strong monovalent strong cation resin cation resinNonsucrose loading 20 10 lbs/ft³ resin/day Step time (time between 30 10switching valves to new position along the loop), minutes Water/feed 2.05.6 volume ratio Extract/raffinate 0.5 0.2 volume ratio Continuous basicinternal 0.29 0.35 circulation rate, BV/hr *The raffinate from SMB A wasincreased in concentration to 60% DS (dissolved solids) prior to beingfed to SMB B

The following results were obtained:

SMB A SMB B Feed % sucrose on D.S. 60.4 69.0 (dry substance) Feed %betaine on D.S. 6.1 0.3 Extract % sucrose on D.S. 2.2 97.1 Extract %betaine on D.S. 50.0 0

Of particular note is the almost complete elimination of betaine acrossSMB A. Betaine is representative of the normally difficult to removetrailing small organic compounds in the system. As a result of theirelimination, SMB B easily produces a very high final sucrose fraction(extract).

As presently understood, displacement chromatography induced on SMB Aimproves the recovery of the sucrose and the small organic molecules intheir respective fractions. For sugar beet and sugar cane syrupapplications, the displacement chromatography mechanism will take effectover several cycles, operated within parameters such as those indicatedin EXAMPLE 1.

EXAMPLE 2

FIG. 3 illustrates the initial approach to equilibrium observed in SMB Aprior to transition to displacement chromatography in a test run. Beetmolasses is the feed material. In FIG. 3, “% dissolved solidsconcentration” is indicated on the vertical axis while “% of internalpath length through the SMB” is indicated on the horizontal axis. Thisprofile is in continuous circulation around the separator path. Underthese conditions, the trailing area, 23, contains small organiccompounds such as betaine and invert sugars; it also contains sucrose,which is in its conventional location for elution chromatography. Theleading edge, 24, contains ash, poorly separated sucrose and highmolecular weight compounds. As operation continues, the trailing smallorganic compounds begin to inventory. Although the specific time totransition is variable, after approximately 20-25 complete cycles, the %dissolved solids profile illustrated by FIG. 3 is observed to make arather abrupt transition to the % dissolved solids profile illustratedby FIG. 4. This transition typically occurs over about 8 cycles. Atrailing plateau, 25, forms from the inventory of small organiccompounds and acts as a chromatographic displacer. At the end of thetransition, the cumulative small molecule % on dissolved solids internalto the SMB is at a very high level compared to the concentrationmeasured in the feed material. This differential inventory of thetrailing small organic compounds causes the displacement mechanism totake effect. The sucrose is forced by the displacement mechanism to moveinto the leading edge area, 26. As a result, the sucrose is recovered ata level of 99% or greater in the leading edge raffinate and the trailingsmall organic molecules are recovered at a level of 90% or greater inthe extract. The system remains in this equilibrated, continuousoperating mode as long as feed material is provided—typically forseveral weeks or months.

In addition to the sudden change in the characteristics of theseparation profile, the practice of this invention results in a decreasein the overall resin bed pressure drop by a factor of about 0.5. Thisdecrease is due to the developed displacement front's pushing thesucrose conventionally found in the trailing area into the leading area,from which a large amount is expelled from the system. No change inoperating parameters is required to cause the expulsion of the sucrose.It is an unattended effect which takes place as the transition todisplacement occurs. Monitoring the total system pressure drop istherefore a useful way of determining the period of transition fromelution to displacement chromatography. Prior to this invention, SMBsystems have been observed to undergo ever increasing pressure drop asthe systems fill with solids during equilibration. They do not undergo asudden pressure drop collapse as is observed in the practice of thisinvention.

The displacement front characteristic of this invention may be formedmore quickly by temporarily adding previously stored or recycled smallorganic molecule fraction (extract from SMB A) to the feed mixture. Thisexpedient will allow the small molecule components to more quicklyinventory inside the system. It is also possible more quickly to formthe displacement front by first loading previously stored small organicmolecule fraction (extract from SMB A) into the appropriate location ofSMB A. A typical procedure is to adjust the stored or prepared smallmolecule fraction to about 15% DS and fill the column to be extractedwith this material. With reference to FIG. 2, column 3 can initially befilled with the small molecule material prior to starting the system'soperation. In addition to decreasing the time required forequilibration, such procedures can be helpful when separating highpurity sucrose mixtures which may require a lengthy inventory period dueto the low ratio of displacer molecules to sucrose.

Both the feed addition and the column loading approaches taught by thisdisclosure are applicable to the chromatographic separation of sugarbeet or sugar cane raw juice as described in U.S. Pat. No. 5,466,294.

Another characteristic of the displacement process of this invention isthat the raffinate product from SMB A can be of relatively low sucrosepurity. The reason for this beneficial consequence is that thenonsucrose in this raffinate is comprised of materials which are easilyseparated in SMB B. With the displacement process, typical raffinatepurities from SMB A can be as low as 65% to 75% sucrose on dissolvedsolids. This range of raffinate purities from SMB A is sufficient easilyto obtain sucrose fraction purities greater than 95% from SMB B.

Unlike conventional elution chromatography, the displacementchromatography mode of this invention is caused by overloadedconditions. Accordingly, a much lower use of eluent is required. Inordinary elution chromatography a decreased use of eluent, is actuallydetrimental to separation efficiency. It is recommended that the ratioof water volume to sucrose mixture volume be in the approximate range of1.0 to 3.0 for 60% dissolved solids molasses. This range can easily becalculated for molasses with other initial % dissolved solids. Tocalculate other cases, the total water in the water stream and the totalwater in the molasses stream are summed and adjusted to a constant. Forexample, water to feed volume ratio will change as the molasses %dissolved solids changes in a specific instance as follows:

Feed % dissolved solids Water to Feed volume ratio

Feed % dissolved solids Water to Feed volume ratio 60 2.0 50 1.4 40 0.83

Another variable which influences development of the displacement frontof small molecules is the volume ratio of extract to raffinate. Removingonly small fractions of extract helps cause the inventory effect tooccur. Extract-to-raffinate volume ratios of about 0.2 to 0.6 aresufficient. Larger volume ratios can cause the small organic moleculeinventory in the chromatographic separator to decrease and thereforereturn the separation mechanism to the less efficient elution mode.

The efficiency of operation of SMB A is influenced by the measure ofnonsucroses loaded per unit of separation resin. Levels of 20 lbs. to 40lbs. of nonsucrose fed per cubic foot of separation resin/day have beenfound to be appropriate to provide an efficient loading condition forthe displacement mechanism. Less loading, while operable, generallyconstitutes an inefficient use of resin.

EXAMPLE 3

The cumulative contents of SMB A were collected to determine the extentof differential inventory of low molecular weight organic compoundswhich cause the displacement effect. For this test, sugar beet molasseswas fed to SMB A until the separation profile in FIG. 3 collapsed to theequilibrated profile in FIG. 4. The operation was stopped, and allsolids contained in the system were rinsed to a single container.Betaine was used as a molecule representative of the displacementcomponents (trailing small organic molecules). Analysis demonstratedthat the cumulative betaine in the equilibrated SMB A was 26.1% ondissolved solids. However the feed material consisted of only 7.3%betaine on solids. Therefore, this result was understood to demonstratea large differential inventory of small organic molecules waseffectively forced upon the internal equilibrated solids profile withinSMB A. The displacement effect of this invention is attributable to thislarge inventory.

What is claimed is:
 1. In a simulated moving bed process for therecovery of a first product fraction predominating in a first productspecies from a liquid mixture containing said first product species inadmixture with a second byproduct species, wherein a feed stock isintroduced to a recycle stream circulating through a partitioned bed ofresin, raffinate is withdrawn from said recycle stream downstream fromthe introduction of said feed stock, eluent is introduced to saidrecycle stream downstream from the withdrawal of said raffinate, andextract is withdrawn from said recycle stream downstream from theintroduction of said eluent, the improvement comprising: establishing afirst continuous simulated moving bed system in which: a first feedstock, comprising a relatively large amount of said first productspecies and a relatively small amount of said second byproduct species,is introduced into a first recycle stream circulating through a firstpartitioned bed of resin; a first raffinate, comprising separated saidfirst product species and containing other contaminant species, iswithdrawn from said first recycle stream downstream from theintroduction of said first feed stock; eluent is introduced to saidfirst recycle stream downstream from the withdrawal of said firstraffinate; and a first extract, comprising separated said secondbyproduct species, is withdrawn from said first recycle streamdownstream from the introduction of said eluent; and establishing asecond chromatographic separation procedure in which said firstraffinate is contacted by a second resin bed to produce: a secondraffinate, comprising said other contaminant species from said firstraffinate; and a second extract, predominating in said first productspecies from said first raffinate at a high purity.
 2. An improvementaccording to claim 1, wherein said second chromatographic separationprocedure is a second simulated moving bed system in which: a secondfeed stock, comprising said first raffinate, is introduced into a secondrecycle stream circulating through a second partitioned bed of resin;said second raffinate is removed from said second recycle streamdownstream from the introduction of said second feed stock; eluent isintroduced to said second recycle stream downstream from the withdrawalof said second raffinate: and said second extract is withdrawn from saidsecond recycle stream downstream from the introduction of said eluent.3. An improvement according to claim 2, wherein: said first continuoussimulated moving bed system is operated such that: a first feed stock,comprising sucrose and nonsucroses, is introduced into said firstrecycle stream circulating through said first partitioned bed of resin;a first raffinate, comprising separated said sucrose, ash and highmolecular weight compounds, is withdrawn from said first recycle streamdownstream from the introduction of said first feed stock; water isintroduced to said first recycle stream downstream from the withdrawalof said first raffinate; and a first extract, comprising separated saidnonsucroses, is withdrawn from said first recycle stream downstream fromthe introduction of said water; and said second simulated moving bedsystem is operated such that: a second feed stock, comprising said firstraffinate, is introduced into said second recycle stream circulatingthrough said second partitioned bed of resin; a second raffinate,comprising ash and high molecular weight compounds separated from saidfirst raffinate, is removed from said second recycle stream downstreamfrom the introduction of said second feed stock; water is introduced tosaid second recycle stream downstream from the withdrawal of said secondraffinate; and a second extract, comprising sucrose, is withdrawn fromsaid second recycle stream downstream from the introduction of saidwater.
 4. A simulated moving bed process for the recovery of sucrosefrom a raw sugar solution, comprising: establishing and operating acontinuous simulated moving bed such that: a feed stock, comprisingsucrose and nonsucroses, is introduced into a recycle stream circulatingthrough a partioned bed of resin; a raffinate, comprising separated saidsucrose, ash and high molecular weight compounds, is withdrawn from saidrecycle stream downstream from the introduction of said feed stock;water is introduced to said recycle stream downstream from thewithdrawal of said raffinate; and an extract, comprising separated saidnonsucroses, is withdrawn from said recycle stream downstream from theintroduction of said water; and treating said raffinate to recover ahigh purity sucrose product; wherein said continuous simulated movingbed is operated through a plurality of cycles under conditions whichbuild up an inventory of low molecular weight nonsucroses, whereby toeffect an equilibrated separation profile in said recycle stream inwhich sucrose has been displaced towards a leading edge of saidequilibrated separation profile.
 5. A process according to claim 4,wherein said raffinate is introduced as a second feed stock into asecond recycle stream circulating through a partitioned bed of resin ofa second simulated moving bed in which: a second raffinate, comprisingash and high molecular weight compounds separated from said second feedstock, is removed from said second recycle stream downstream from theintroduction of said second feed stock; water is introduced to saidsecond recycle stream downstream from the withdrawal of said secondraffinate; and a second extract, comprising sucrose, is withdrawn fromsaid second recycle stream downstream from the introduction of saidwater.
 6. In a simulated moving bed process for the recovery of sucrosefrom a raw sugar solution in which a feed stock is introduced to arecycle stream circulating through a partitioned bed of resin, raffinateis withdrawn from said recycle stream downstream from the introductionof said feed stock, water is introduced to said recycle streamdownstream from the withdrawal of said raffinate, and sucrose iswithdrawn from said recycle stream downstream from the introduction ofsaid water, the improvement comprising: establishing as a firstchromatographic separation procedure, a simulated moving bed in which: afirst feed stock, comprising sucrose and nonsucroses, is introduced intoa recycle stream circulating through a partitioned bed of resin; a firstraffinate, predominating in separated said sucrose, ash and highmolecular weight compounds, is removed from said recycle streamdownstream from the introduction of said first feed stock; water isintroduced to said recycle stream downstream from the withdrawal of saidfirst raffinate; and a first extract, comprising separated saidnonsucroses, is withdrawn from said recycle stream downstream from theintroduction of said water; and establishing a second chromatographicseparation procedure in which said first raffinate is contacted by asecond resin bed to produce: a second raffinate, comprising ash and highmolecular weight compounds from said first raffinate; and a secondextract, predominating in sucrose from said first raffinate at a highpurity.
 7. An improvement according to claim 6, wherein said simulatedmoving bed is operated to create a high steady state inventory of smallorganic molecules in said recycle stream, whereby to displace sucrosetowards a leading edge of a separation profile of said recycle stream.8. A simulated moving bed process for the recovery of a first productfraction predominating in a first product species from a liquid mixturecontaining said first product species in admixture with a secondbyproduct species, comprising: establishing a continuous simulatedmoving bed system in which: a feed stock, comprising a relatively largeamount of said first product species and a relatively small amount ofsaid second byproduct species, is introduced into a recycle streamcirculating through a partitioned bed of resin, said recycle streambeing characterized by a separation profile; a raffinate, comprisingseparated said first product species and containing other contaminantspecies, is removed from said recycle stream in a vicinity of a leadingedge of said separation profile, downstream from the introduction ofsaid feed stock; eluent is introduced to said recycle stream downstreamfrom the withdrawal of said raffinate; and an extract, comprisingseparated said second byproduct species, is withdrawn from said recyclestream downstream from the introduction of said eluent in a vicinity ofa trailing edge of said separation profile; and accumulating into saidrecycle stream sufficient displacer species to displace said firstproduct species toward the leading edge of said separation profile ofsaid recycle stream.
 9. A process according to claim 8 in which saiddisplacer species is said second byproduct species.
 10. In the operationof a continuous simulated moving bed system to separate the componentsof a feed stock, wherein a resin bed is divided into a series ofdiscrete vessels, each of which functions as a zone within a circulationloop through which is maintained a relatively large-volume continuousrecycle stream, and a manifold system connects the vessels and directsin appropriate sequence to each such vessel relatively small-volumestreams of feed stock and eluent, respectively, and from each vesselrelatively small-volume streams of extract and rafinate, respectively,whereby to develop a circulating inventory of chromatographicallyseparated chemical species characterized by a separation profileincluding an intermediate region occupied by a commercially valuablephase bounded by a trailing region and a leading region, the improvementwhich comprises operating said system to establish a high inventory of atrailing separated species, whereby to displace a commercially valuableseparated species into said leading region, and collecting saidraffinate stream from said leading region of said separation profile.